With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any existing or future Western European, Chinese, Japanese, Indian or other SPS such as the Galileo program.
The most commonly used system in the U.S., the GPS system, was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of four satellites are visible at any location on the surface of the earth except in the polar region at all times. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver typically has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The signals from the navigational satellites are modulated with navigational data at 50 bits/second (i.e. 1 bit/20 msec). This navigational data consists of ephemeris, almanac, time information, clock and other correction coefficients. It also contains information about ionospheric corrections, satellite constellation health and other associated information needed to correctly and reliably compute receiver's position and time.
The almanac and ephemeris are used in the computation of the position of the satellites at a given time. The almanacs are valid for a longer period of six days or much longer but provide a less accurate satellite position and Doppler compared to ephemeris. The accuracy of the satellite position and velocity degrades with older almanac data. For example, the accuracy of satellite position and velocity computed with one week old almanac would be better than that computed using one month old almanac. Therefore, almanacs are not used when a accurate position fix is required. On the other hand, the accuracy of the computed receiver position depends upon the accuracy of the satellite positions which in-turn depends upon the age of the ephemeris. The use of current ephemeris results in better and faster position estimation than one based on non-current or obsolete ephemeris. Therefore, it is necessary to use current ephemeris to get a fast and accurate receiver position fix.
A GPS receiver may acquire the signals and estimate the position depending upon the already available information. In the “hot start” mode the receiver has current ephemeris and further the approximate position and time are known. In another mode known as “warm start” the receiver has non-current ephemeris (or does not have ephemeris) and the initial position and time are known less accurately than in the case of previous “hot start.” In the third mode, known as “cold start,” the receiver has no knowledge of approximate position, time or ephemeris. As expected, the “hot start” mode results in low Time-To-First-Fix (TTFF) while the “warm start” mode which if it has non-current ephemeris may use that ephemeris or the almanac resulting in longer TTFF due to non availability of current ephemeris. The “cold start” takes still more time for the first position fix as there is no data available to aid signal acquisition and position fix.
Therefore, it is desirable to keep the ephemeris in the receiver current for a fast TTFF. Current ephemeris also helps when the received signal is weak and the ephemeris can not be downloaded. Some issued patents teach receiving the ephemeris through an aiding network or remote server instead of from an orbiting satellite (i.e. an assisted GPS server providing current ephemeris obtained from reference receivers with full view of the sky or server generated extended ephemeris or SGEE). However, this approach results in higher cost and requires additional infrastructure. Another approach to keeping ephemeris current, without using a remote server, is to automatically download it from satellites in the background, such as described in U.S. Pat. No. 7,436,357.
Some commercially available products such as SiRF InstantFixII from SiRF Technologies of San Jose, Calif. use extended ephemeris to improve start-up times without requiring network connectivity (i.e. client generated extended ephemeris or CGEE). With one observation of a satellite, SiRFInstantFixII accurately predicts that satellite's position for up to three days—removing the need to download satellite ephemeris data at subsequent start-ups—resulting in full navigation in as little as five seconds, and with routine 7 meter accuracy. Moreover, such extended ephemeris products not only start tracking satellites and navigating more quickly, they can do it using signals much weaker than those needed to obtain satellite ephemeris data the traditional way, removing the barrier that often blocks successful navigation under tough GPS signal conditions.
Nevertheless, some challenges remain. For example, the orbits of GPS satellites are inclined at 55 degrees, which causes the line-of-sight (LOS) between a receiver and the satellite to be closer to the horizon when a receiver is nearer to the Earth's poles. This can cause reception and tracking problems, especially with nearby objects such as buildings or trees. Also in situations where there is significant blockage of sky (for example, small streets with tall buildings on all sides) sufficient number of GPS satellites may not be available to make an accurate position computation.
These and similar problems could be lessened if it were possible to use satellites from other systems, additionally or alternatively to GPS satellites, when performing navigation for a single user. For example, satellites from the GLONASS system can provide better LOS to receivers when they are at higher latitudes. Moreover, the availability of satellites with different orbits than GPS satellites also provides more flexibility in “urban canyon” and other types of difficult LOS environments. However, GLONASS and other satellite systems have different data formats, which do not easily allow integration with GPS solutions, including extended ephemeris technologies.
Accordingly, a need remains for an accurate and reliable way to use satellite state and/or ephemeris information when performing satellite-based navigation with satellites from two or more systems, including performing extended ephemeris with such mixed systems.